This invention relates to a process for the preparation of a stabilized aqueous alkali metal-2-hydroxy-4,6-dichloro-s-triazine (AHDT) solution and use thereof.
The alkali metal and alkaline earth metal salts of 2-hydroxy-4,6-dichloro-s-triazine, especially the sodium salt thereof (NHDT), are preferred hardening agents for gelatinous materials which are used in particular for photographic purposes. Accordingly, there are numerous known methods of preparing these triazine derivatives, and also of hardening solutions containing them.
However, the stability of the hardening solutions remains problematic. This is because both during synthesis and storage of the carbonate-containing hardening solutions, and often during their use, carbon dioxide is formed and the triazines are hydrolyzed to the corresponding monochlorinated compounds. Another particular disadvantage connected with the formation of carbon dioxide is that the gas generates bubbles in the photographic layers produced with the hardening agent.
To overcome these difficulties, it has been suggested in the past that the aqueous hardening solutions be buffered with a water-soluble borate (DE-OS 28 20 108). According to the DE-OS 29 10 726, the buffering effect can be enhanced by means of an alkali metal hydroxide and possibly by addition of water-soluble organic solvents, for example acetone, which are inert under the reaction conditions. The amount of alkali borate used in this case is only 0.02 to 2 moles per mole AHDT.
It has been found in practice, however, that neither the sole measure of buffering with borates nor the additional use of alkali metal hydroxides solves the problem of carbon dioxide formation. A further disadvantage of the known synthesis method is the very slow formation of the respective alkali metal-2-hydroxy-4,6-dichloro-s-triazine (AHDT).
The object of this invention was thus to provide a process for the preparation of a stabilized aqueous alkali metal-2-hydroxy-4,6-dichloro-s-triazine (AHDT) with which, in particular, undesirable formation of CO2 is suppressed both during the synthesis reaction and during storage and use of the prepared AHDT solution. In addition, the preparation process should be easy and quick to carry out.
This object was established by
(a) providing an aqueous alkali hydrogencarbonate and/or acetone starting solution,
(b) adding cyanuric chloride and an alkali lye under such conditions that the temperature does not rise above 25xc2x0 C. and the pH does not drop below 5.5;
(c) adding an alkali salt of an organic acid for the case that in (a) a hydrogencarbonate-containing solution was used, or adding an alkali hydrogen carbonate for the case that in (a) exclusively acetone was used;
(d) adding a stabilizer selected from an alkali borate and/or an alkali (hydrogen) phosphate and/or an alkali hexacyanoferrate (II), and
(e) if necessary adjusting the pH to the desired value with acid.
In a preferred embodiment, the process is conducted at temperatures between 5 and 40xc2x0 C. in the following way:
(a) providing an aqueous 0.1 to 1.0 wt. % alkali hydrogencarbonate solution and/or a 0.5 to 5 wt. % acetone solution as starting solution, followed by
(b) adding 8 to 16 wt. % cyanuric chloride, expressed in terms of the solution from (a); wherein during the addition of cyanuric chloride, 1 equivalent expressed in terms of cyanuric chloride of a 25 to 50 wt. % alkali lye, is added at a temperature not exceeding 25xc2x0 C. to the suspension in such a manner that the pH of the reaction mixture does not drop below 5.5;
(c) subsequently adding, during a stirring period of 2 to 30 hours, 0.1 to 1.5 wt. ‰ of an alkali lauryl sulfate for the case that in (a) a hydrogen-carbonate-containing solution was used, or of 0.5 to 1.0 wt. % of alkali hydrogen carbonate for the case that in a) exclusively acetone was used, both weight precentages being expressed in terms of the overall solution;
(d) finally, adding 0.3 to 1.2 wt. %xe2x80x94expressed in terms of the overall solutionxe2x80x94of an alkali borate and/or of dialkali hydrogenphosphate and/or of potassium hexacyanoferrate (II), and
(e) if necessary, adjusting the pH to a value in the range from 8 to 9.5 with an acid.
Surprisingly, it was found that very small amounts of acetone or even the complete absence of acetone have no deleterious effect at all on the course of the reaction or on the reaction time. In view of the known prejudicexe2x80x94contained, for example, in the DE-OS 29 10 726xe2x80x94this was in no way to be expected. Moreover, particularly the use of alkali hydrogen carbonate permits wide variation in the way the process is conducted, it being possible at any time to quantitatively buffer the theoretically forming carbon dioxide.
Due to the use of alkali hydrogen carbonate, the process according to the invention indeed permits a wider temperature range, for example from 5 to 40xc2x0 C., although temperatures between 10 and 20xc2x0 C. are preferred. However, these relatively low temperatures have no deleterious effect whatsoever on the speed of reaction of the overall process.
For the process described here, sodium and potassium are preferred as alkali metals and have proved highly suitable. Using the same alkali metal for all steps of the process has the advantage that one obtains the pure sodium- or pure potassium-2-hydroxy-4,6-dichloro-s-triazine; however, it is also possible to use mixtures of these products.
The respective alkali hydrogencarbonate can be provided as aqueous starting solution, in which case a 0.5 to 0.8 wt. % solution is preferred. Alternatively, it can be added later on in the process, namely in step (c), if in step (a) aqueous acetone solution was used exclusivelyxe2x80x94according to the invention preferably in an amount of 1.0 to 2.0 wt. %. It is also possible, however, to start with a mixture consisting of alkali hydrogencarbonate and acetone in arbitrary proportions, provided amounts are within the range claimed.
The choice of alkali lye for step (b) will depend on which alkali hydrogencarbonate is used: if NaHCO3 is used in step a) and/or step (d), it is preferable to add NaOH, whereas if KHCO3 is used, it is preferable to add potassium lye in step (b).
All in all, there are thus no strict parameters governing the way in which the process of the invention is conducted. To optimize the course of the reaction, however, it is recommended that the amount of cyanuric chloride used in step (b) preferably be in the range from 10 to 12 wt. % expressed in terms of the solution from (a). The reaction sequence is likewise speeded up by selecting a preferred stirring time of 3 to 6 hours in step (c).
Like the alkali lye in step (b), the alkali salt added in step (c)xe2x80x94preferably the alkali salt of a C10-C20 organosulfuric acid, in particular alkali lauryl sulfatexe2x80x94can be selected specifically to match the respective alkali hydrogencarbonate and thus to ensure a pure product.
As already explained, the principal objective of developing the process claimed here was to suppress the undesirable formation of CO2. This objective is achieved among other things by the addition of stabilizing and buffering substances in step (d) of the process according to the invention, preference being given to the alkali borates Na2B4O7 or K2B4O7, their respective pentahydrates or decahydrates or mixtures thereof; as representatives of the alkali (hydrogen)phosphates, which are likewise suitable according to the invention, trialkali phosphates, alkali hydrogenphosphates and, in particular, dialkali hydrogenphosphates such as disodium and dipotassium hydrogenphosphate are preferred; the choice may again depend on the alkali hydrogencarbonate selected in each case.
For the most part, the pH does not constitute a limiting parameter for the process claimed. However, to ensure complete hydrolysis of the cyanuric chloride, it may be necessary according to the invention to adjust the pH at the end of step (d) to a value between 8.0 and 9.5 using an acid; wherein a mineral acid is recommended, preferably hydrochloric acid. It is especially beneficial if the pH is in the range from 8.5 and 9.0 for step (e). It is altogether beneficial for the course of the reaction according to the invention if the pH of the reaction mixture in steps (a) to (d) is xe2x89xa77.0.
In cases where step (c) requires addition of organic alkali salt, spent NaHCO3 or KHCO3 can be replaced by adding it at the same time; an excess of alkali hydrogencarbonate has been found to have no deleterious effect whatsoever on the stability of the AHDT solution because any CO2 which may have formed dissolves completely in the nascent sodium- or potassium-2-hydroxy-4,6-dichloro-s-triazine solutions.
Certain requirements for the AHDT solutions prepared according to the process claimed may likewise necessitate that the stabilized solution be additionally clarified. To this end, the invention provides for filtration of the solutions following step (d) or (e). Filtration can be preceded by the addition of activated carbon.
The stabilized aqueous solutions of sodium- or potassium-2-hydroxy-4,6-dichloro-s-triazine are used preferably for hardening gelatin or for textile or paper finishing.
All in all, this invention provides a process which permits unproblematic preparation of stabilized aqueous AHDT solutions. The use of alkali hydrogencarbonates as co-stabilizers has proved insofar beneficial as the entire duration of the process is reduced drastically as a result. It is also possible, if desirable, to dispense entirely with acetone.
Stabilized aqueous alkali metal-2-hydroxy-4,6-dichloro-s-triazine solutions containing less than 4.0 wt. %, preferably less than 2.5 wt. % acetone, are also subject matter of the invention. Solutions which contain no acetone or other organic solvents are especially preferable. It is beneficial if the solutions have a pHxe2x89xa77.0, and especially beneficial if the pH is in the range from 8.0 to 9.5. In addition, the solutions preferably show a pH stability (pHxe2x89xa77.0 during storage at room temperatur (e) of at least 100 days or, especially preferable, of at least 250 days.
The following examples illustrate the advantages of the invention.